The prior art contains a number of references to fluidic lens systems. A notable example is provided by those based on the electro-wetting effect (see, e.g. Bruno Berge, et al., “Lens with variable focus”, PCT Publication No. WO 99/18456). In that system, a lens-like volume of one refractive liquid is separated from its surroundings on at least one side by another immiscible refractive liquid. Although this yields a conveniently compact system, it is difficult to provide enough refractive index difference between the two liquids to provide adequate light-ray bending ability. A refractively superior system has also been demonstrated (see J. Chen et al., J. Micromech. Microeng. 14 (2004) 675-680) wherein only one lenticular body is provided, bounded on at least one side by an optically clear, compliant membrane. In that system, the refractive power of the lens is controlled by pumping in or out a controlled amount of fluid, thereby changing the curvature of the bounding membrane. Although improved, that system still suffers from the disadvantage that the pressurized fluid source is located remotely. This makes the form-factor of the whole system inconvenient.
One type of fluidic lens described, e.g., U.S. Patent Application Publication 20070030573 incorporates a fluid-filled chamber which is capable of squeezing transparent fluid into a lens which is centrally-disposed and includes one or more elastic-membranes. Actuation of the lens is accomplished by pressurization of the fluid. This pressurization, in turn, causes the membranes to bulge, thereby controllably altering the optical power of the lens. The elastic energy of the membranes may provide a restoring force which may counteract the actuation force. Once the actuation force is diminished, the restoring force of the membrane may contribute to the restoration of the membrane to its original or non-actuated state.
The membranes incorporated in fluidic lenses may be typically chosen for their elastic properties. That is, a membrane material having low elastic modulus is often desirable since it may reduce the force required by the actuator. One elastomeric membrane material commonly used in fluidic lenses is polydimethylsiloxane (PDMS) Sylgard 184 manufactured by Dow Corning. This material is used for its optical and mechanical properties. These properties may include its Young's modulus, which can range from about 0.05 to 2 MPa, optical transparency, ease of fabrication and replication of small features and surface wetability.
However, while many of the properties of PDMS and similar elastomers may be desirable in fluidic lenses, the value of the Young's modulus presents an inherent disadvantage for certain applications. As used herein, the word “modulus” may be interpreted to mean “Young's modulus” or “elastic modulus”. That is, for a lens having a membrane with a sufficiently low modulus, the lens may be susceptible to disturbances, such as instabilities in focus and tilt due to forces of acceleration, and aberrations, such as coma, which may be due to gravitational forces.
One solution to this problem is to pre-tension, or stretch, the membrane during fabrication of the lens, thereby increasing the “effective modulus” (or “effective stiffness”). However, disadvantages with pre-tensioning the membrane may include a slow long-term relaxation of the membrane that lowers its effective stiffness, sensitivity to small non-uniformities in tension of the membrane resulting in optical aberrations across the dynamic range of the lens, and the possible appearance of bulk defects in the membrane upon tensioning that result in optical scattering. Further, as the lens aperture is increased, the pre-tensioning must generally be increased in order to avoid the effects of acceleration and gravity, thereby increasing the sensitivity of the lens to these undesirable effects. Similarly, as the optical power (i.e., the radius of curvature of the membrane) is increased, the pre-tensioning of the membrane must be increased in order to avoid these effects.
Another inherent disadvantage of PDMS is its permeability, or inability to effectively block the passage of some gases and fluids. Such permeability may result in air bubbles developing in, or fluid leaking out of, the lens. These effects can diminish the durability, lifetime, optical quality, dynamic range and other performance properties of the lens. Some approaches to solving this problem may include coating the PDMS with a high-barrier material or increasing the thickness of the membrane. However, these approaches can result in disadvantageous effects such as increasing the complexity of fabrication, optical scatter and loss, and aberrations.
Thus, there is a need in the art, for a fluidic lens that overcomes the above disadvantages.